I wouldn't have a clue how to rip soundtracks from Xbox games, you'd probably need to obtain a dashboard hack patch and install it, run a crossover cable from the Xbox to your Mac, then stream all the data to your HD, and sort out the .xma, .grt, and .nda files. Well, that was the only info I found via Google, I don't even have an Xbox!
The easier alternative would be to just hook up the Xbox's audio out to the Mac and then record it the ol' fashioned way.